Lithium ion secondary batteries are characterized by their small size and large capacity and are widely used as power sources for electronic devices such as mobile phones and notebook computers, and have contributed to the improvement of the convenience of portable IT devices. In recent years, attention has also been drawn to the use in large-sized applications such as drive power supplies for motorcycles and automobiles, and storage batteries for smart grids. As the demand for lithium ion secondary batteries has increased and they are used in various fields, batteries have been required to have characteristics, such as further higher energy density, lifetime characteristics that can withstand long-term use, and usability under a wide range of temperature conditions.
In general, carbon-based materials have been used for a negative electrode of the lithium-ion secondary battery, but in order to increase the energy density of the battery, the use of silicon-based materials having a large capacity of absorbing and desorbing lithium ions per unit volume has been studied for a negative electrode. However, the silicon-based material deteriorates due to expansion and contraction that are repeated by charging and discharging lithium, and therefore have a problem in the cycle characteristics of the battery.
Various proposals have been made to improve the cycle characteristics of the lithium ion secondary battery using a silicon-based material for a negative electrode. Patent Document 1 discloses a method of improving the charge and discharge cycle life of a nonaqueous electrolyte secondary battery by mixing silicon oxide with elemental silicon and further covering its periphery with amorphous carbon to relax the expansion and contraction of the electrode active material itself. Patent Document 2 discloses that, by specifying the size ratio of silicon oxide particles and graphite particles in a negative electrode comprising silicon oxide and graphite, the silicon oxide particles are disposed within spaces formed by graphite particles to suppress the change in the volume of the entire negative electrode even when the silicon oxide expands, and thus, the deterioration of the cycle characteristics can be suppressed.